Oilpan: Don't trace the RefPtr for the CSSCursorImageValue workaround. It is trace via the RefCountedGarbageCollected persistent.

We are correctly hitting an UNREACHABLE when tracing a RefPtr with
enable_oilpan=1 on the bots.

R=haraken@chromium.org, wibling@chromium.org, oilpan-reviews@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/177413009

git-svn-id: svn://svn.chromium.org/blink/trunk@168459 bbb929c8-8fbe-4397-9dbb-9b2b20218538